Steel sheets such as zinc-based plated steel sheets or aluminum-based plated steel sheets which are further treated by chromating thereon in a treatment solution containing main ingredients of chromic acid, bichromic acid, or salt thereof have widely been used for the steel sheets for household electric appliances, the steel sheets for buildings, and the steel sheets for automobiles aiming to improve the corrosion resistance (resistance to white rust and resistance to red rust). The chromating is an economical treatment method to provide excellent corrosion resistance and to be conducted relatively easily.
Although chromating uses hexavalent chromium which is a regulated substance for pollution prevention, the hexavalent chromium is treated in a closed system in the treatment process, thereby being completely reduced and recovered, and is not emitted to natural environment. Furthermore, the sealing action of organic coating allows very little the chromium to elute from the chromate coating so that environment and human body are substantially not contaminated by the hexavalent chromium. From the viewpoint of recent global environmental concern, however, the movement of independently decreasing the use of heavy metals including hexavalent chromium has been enhanced. In addition, to prevent the environmental contamination of discarded shredder-dust of industrially wasted products, there has been begun a movement to eliminate or to minimize the heavy metals in the products as far as possible.
Under the situation, there are introduced many treatment technologies to prevent the generation of white rust on zinc-based plated steel sheet without applying chromating, or what is called the “chromium-free technology”. An example of the chromium-free technology is a method using a solution of an inorganic compound, an organic compound, an organic polymer material, or a combination thereof, adopting immersion, application, electrolytic treatment, and the like to form a thin film on the zinc-based plated steel sheet.
Typical related technologies are the following.
(1) A method of forming coating by immersing the steel sheet in a treatment solution containing a polyphenol carboxylic acid such as tannic acid and a silane coupling agent, or by applying the treatment solution onto the steel sheet, (for example, Patent Document 1 and Patent Document 2).
(2) A method of forming coating using a treatment solution containing an organic resin blended with a polyphenol carboxylic acid such as tannic acid or a phosphoric acid compound, (for example, Patent Documents 3 through 6).
(3) A method of applying a coating prepared by blending an organic resin and a silane coupling agent, (for example, Patent Documents 7 through 13).    Patent Document 1: Japanese Patent Laid-Open No. 7-216268    Patent Document 2: Japanese Patent No. 2968959    Patent Document 3: Japanese Patent Laid-Open No. 8-325760    Patent Document 4: Japanese Patent Laid-Open No. 2000-34578    Patent Document 5: Japanese Patent Laid-Open No. 2000-199076    Patent Document 6: Japanese Patent Laid-Open No. 2000-248380    Patent Document 7: Japanese Patent Laid-Open No. 11-106945    Patent Document 8: Japanese Patent Laid-Open No. 2000-319787    Patent Document 9: Japanese Patent Laid-Open No. 2000-248384    Patent Document 10: Japanese Patent Laid-Open No. 2000-178761    Patent Document 11: Japanese Patent Laid-Open No. 2000-199076    Patent Document 12: Japanese Patent Laid-Open No. 2000-281946    Patent Document 13: Japanese Patent Laid-Open No. 2000-14443
As for the above method (1), there is a method of treatment in an aqueous solution of a polyphenol carboxylic acid, a silane coupling agent, and further a metal ion. An example of the treatment method is the one described in Patent Document 1. Since, however, the treatment method has a drawback of failing to attain sufficient corrosion resistance, though favorable adhesion of coating is available.
As for the above method (2), Patent Document 3, for example, discloses a method of treatment using a treatment solution containing a polyphenol carboxylic acid, an organic resin, and a metal ion. In addition, Patent Document 4 discloses a method of treatment conducted by immersing the steel sheet in a treatment solution containing an organic resin and a phosphoric acid compound, or by applying the treatment solution onto the steel sheet, followed by drying the applied solution. The protective coating formed by these treatment solutions cannot attain strong corrosion resistance which is attained by the chromating, though the protective coating contributes to the improvement in corrosion resistance to some degree.
As for the above method (3), Patent Documents 8 and 9 disclose a steel sheet having a coating which contains an organic resin, a silane coupling agent, and further a thiocarbonyl compound, a phosphoric acid compound, and a vanadium compound. Since, however, the organic resin is polyurethane or acrylic-olefin resin, the corrosion resistance is not satisfactory. Patent Document 11 discloses a steel sheet having a coating of acid-modified epoxy resin. Patent Document 10 discloses a steel sheet having a coating prepared by blending a resin which contains copolymerization ingredients of monomer containing hydroxyl group, carboxyl group, glycidyl group, and phosphoric acid group, with a silane coupling agent and a phosphoric acid compound. Both of these coatings do not give sufficient corrosion resistance. Patent Document 7 discloses a steel sheet having a coating containing a polyvinylphenol derivative, a silane coupling agent, and an etchant such as phosphoric acid. The coating also cannot give sufficient corrosion resistance. Patent Document 12 discloses a steel sheet having a coating containing an organic resin and an etchant, and Patent Document 13 discloses a steel sheet having a coating containing an organic resin and a silane coupling agent. Both of these Patent Documents do not give detail description, and their coatings have insufficient corrosion resistance.